Codes P0171 and P0174?

A reader asks, "My Check Engine Light came on. I pulled the codes and according to AlldataDIY, I find P0171 Fuel Trims Too Lean Bank 1 and P0174 Fuel Trims Too Lean Bank 2. What would make the engine lean on both sides?

Well, we can use some deductive reasoning here. A lean burning engine either has too much air or not enough fuel. If the condition is present on both sides of the engine, then we have to look at things that affect both sides.

Fro example, a restricted fuel injector would only affect the bank it is on. An intake manifold leak, likewise usually will only affect the side of the engine that it is on.

Things That Affect The Whole Engine

• Fuel Filter. A clogged fuel filter would starve the whole engine resulting in both banks being equally lean. However, a restricted fuel filter would probably have other symptoms like a loss of power at higher speeds.
• Fuel Pump. A worn electric fuel pump that is turning too slowly might not supply enough fuel to maintain a proper mixture. Again other symptoms should present. Power loss, hard starting, etc.
• PCV System. Here's the culprit I run into most. Positive Crankcase Ventilation System. On most late model cars and trucks, especially domestic, the PCV valve system uses rubber hoses and elbows to connect the valve to the intake manifold. Oil vapors from the crankcase attack the rubber and cause it to disintegrate over time. A small hole will let a good amount of unmetered air into the system which will affect both banks. This is a big Ford problem. Sometimes moving the hose a bit with the engine running will actually produce a prominent hissing sound to verify a leak.
• Intake Hose Leaks. Between the filter housing and the Throttle Body Assembly is usually a rubber flex hose to get the air into the engine. The Mass Airflow Sensor is usually attached to or very near the filter housing. If there is an air leak after the MAF Sensor, the Powertrain Control Module will have air to deal with that it doesn't know is there. Consequently the amount of fuel injected will be insufficient for the amount of air present, hence a lean condition on both sides of the engine. This is particularly a possibility if someone has recently replaced the air filter and did not tighten all clips and hose clamps.

So simple detective skills can get you headed in the right direction on a lean engine. Good Luck and Happy Wrenching.